> What are the levels of uncertainty associated with back trajectory calculations in HYSPLIT?
What are the levels of uncertainty associated with back trajectory calculations in HYSPLIT?
Some of the FAQ's suggest that running the forward trajectory and backward
trajectory simulators produce answers that will vary by approximately
1/2 the distance between the start and end points on average. Could
this number be interpreted as the uncertainty in the numerical
calculation? On top of that error, do you have an estimate for how far
away from reality you expect the simulator to be on average?
The difference between the start position and end position of a forward-backward
trajectory combination indeed represents the numerical error. However, more
significant error is introduced because the gridded meteorological data file
used for the computation is a discrete representation of a continuous function.
Therefore, depending upon the the spatial and temporal complexity of the
meterological fields, additional error will be introduced if the grid
resolution is not sufficient. Typically a grid resolution of "x" can only
represent wavelengths of "4x". This error will be a function of the
meteorological conditions. Then there is the implicit error introduced by the
assumption that the trajectory (the integration of a single point in space and
time) represents something more than its explcit definition. For instance,
assuming that a single trajectory represents the pollutant flow from a source
to a receptor, when in fact that situation must be represented by many
trajectories to represent the effects of turbulence and temporal variations in
the flow over the duration of emissions and sampling.
